This invention relates to a passive thermal energy storage apparatus storing thermal energy in the form of heat of fusion.
Energy storing systems have been suggested to permit storage of the thermal energy from a suitable source during low peak usage periods or when such energy is available for subsequent use as necessary and, particularly, for storage of solar energy and the like. Energy storing systems may, of course, also be employed for cooling purposes where the cool night air, cool water or the like is employed as the cooling energy source.
Energy storage systems have employed various materials for storage of the energy, such as (1) solid or packed bed storage, (2) liquid storage and (3) phase change storage.
The packed bed storage system may employ rocks or the like, which have the disadvantage of large volume or space requirement. Liquid storage systems widely employ water as the thermal energy storage medium, which have the disadvantage of requiring relatively high temperature collector sources because the heat is stored only as sensible heat in the storage medium.
Phase change systems use salt hydrates, such as sodium sulphate decahydrate, also generally known as glauber salt, or pentahydrate as the storage medium. The hydrates turn to a liquid when heated and freeze into crystals when cooled to its melting temperature. In one system, the salt is held in a continuously rotating tank with a dynamically controlled means for introducing seed crystals into the tank to initiate the crystallization. The tank is continuously rotated to maintain a uniform water and salt mixture and a bath temperature which is close to wall temperature to prevent undesirable built upon portions of the tank wall rather than at the nucleation center when the seed crystal is introduced. In addition, temperature monitoring or other control means are employed to initiate the nucleation at or below the normal melting temperature. Although such salt has been employed for storage because the storage capacity is substantially greater than water or the like, other phase change materials and associated dynamically controlled release mechanisms have also been suggested.
Improved thermal energy storage systems employing other supercooled storaged mediums in which the energy is stored has been suggested. For example, dynamically controlled and triggered supercoolable material storage systems are also shown in U.S. Pat. No. 3,093,308, which issued to Charles D. Snelling on June 11, 1963 and U.S. Pat. No. 3,952,519, which issued to William K. R. Watson on Apr. 27, 1976. A highly satisfactory triggered system is disclosed in the application of Paul E. Thoma for THERMAL ENERGY STORAGE APPARATUS, filed on Nov. 24, 1976, with Ser. No. 744,695. Generally, in the prior art systems, a supercoolable material is contained within a suitable housing or container in combination with a triggering means for selectively introducing a nucleating device or means into the liquid. The supercoolable material is selected to have a supercooled liquid state and temperature well below the ambient storage temperature. The material is heated to its melting temperature to form a liquid body above the melting temperature. The liquid body may then be allowed to cool to a temperature below the melting temperature, that is to the temperature of the surrounding environment. As the supercoolable liquid cools below the melting temperature, it does not freeze or change state but rather supercools in the liquid state to the temperature of the surroundings, or a higher temperature depending upon the insulation about the container. In the supercooled state, the liquid stores thermal energy as the heat of fusion of the supercoolable liquid. To recover such heat, it is merely necessary to nucleate the liquid, thereby causing a rapid freezing or solidification with a corresponding liberation of the heat stored therein. The material is such that when in the supercooled liquid state, solidification or freezing can be initiated by use of a nucleating device such as a cold finger, a seed crystal, or a sharp pointed object.
Although such dynamically controlled systems provide a means for effective storage of substantial quantities of thermal energy, the systems generally present practical constructional and operational difficulties which have limitedpractical and extensive usage and which has resulted in an on-going investigation and study of improvements and alternatives of such systems.